A variety of applications exist for composite materials comprising particulates held together in a binder matrix. For example, composites made of graphite, glass, boron nitride, silicon carbide, metal, etc., bonded together with a relatively small amount of a thermosetting or thermoplastic polymer have generated considerable interest in recent years for a number of applications. For example, a need exists for low-cost magnetic products such as (1) magnetic cores (e.g., stator cores) for electrical rotating machinery and (2) high strength permanent magnets. Magnetic cores (i.e., flux concentrators) made from polymer-bonded metal (e.g., iron and steel) particles and permanent magnets made from polymer-bonded magnetizable particles have both heretofore been proposed. In this latter regard, permanent magnets have been made from rare-earth-ferromagnetic metal alloy particles held together in a plastic matrix. Early such magnets used samarium-cobalt and other rare earth compounds in a variety of binders. More recently, magnetic compositions containing neodymium and/or praseodymium, iron and boron such as described in U.S. Ser. Nos. 414,936 and 544,728, (both assigned to the assignee of this application), have received considerable attention and these, too, have been bonded together in a polymer matrix. In both cases (i.e., magnetic cores and permanent magnets), high density products having metal loadings greater than about 70% by volume are required to obtain good flux concentration and magnetic strength respectively. Preferably, the metal loading will be greater than about 85% by volume and most preferably about 92% by volume for optimal properties. Generally speaking the higher the metal particle content the better the magnetic or flux-concentrating properties of the part. Hence, the least amount of binder possible is most desirable consistent with the strength requirements, integrity and particle isolation needs of the part.
It is known from a method standpoint to make polymer-bonded magnets, for example, by first directing a stream of molten iron-neodymium-boron alloy onto the perimeter of a rotating chill disk to very rapidly quench the alloy into thin ribbon The thin "melt-spun" ribbon is then ground or comminuted into small particles which are then mixed with a binding agent, placed in the cavity of a conventional punch and die set in a cold press, and compressed/compacted to densify the particle-binder mix and conform it to the shape of the cavity (e.g., disk-shaped). Following compacting, the binding agent is activated either by heating, ultraviolet radiation, or curing in any appropriate fashion which is dictated by the characteristics of the particular binding agent chosen. For example, Gray U.S. Pat. No. 4,558,077, assigned to the assignee of the present invention, employs a thermosetting epoxy comprising polyglycidyl ethers of polyphenol alkanes and a latent imidazole curing agent which is activated by subsequent heating in an oven to cure the binder. Compression molding techniques in heated dies are used with thermoplastic binders. If made from magnetically anisotropically particles, the resulting product is then aligned in a magnetic field prior to subsequent processing. Magnetically isotropic particles, such as Fe-Nd-B, do not require such alignment. Heretofore, such compacting and curing procedures have required the use of heavy compaction presses (i.e., about 5 tons), and relatively long cycle times not to mention additional equipment (e.g., curing ovens), where applicable, and accordingly contribute significantly to the cost of the product being produced.
It would be commercially advantageous to have a simpler, quicker (i.e., on the order of seconds), and less expensive method for making low-binder-content (e.g., less than about 30% by volume) particle-binder composites. It would be especially advantageous to have a quick method for simultaneously compacting and bonding the particles together at significantly lower pressures than heretofore required. It is the principal object of the present invention to provide an improved method for accomplishing the foregoing especially as it relates to the manufacture of magnetic products requiring high metal particle loadings.